The present invention relates to a self-adapting process and device for triggering a triac according to an adjustable conduction angle.
A triac is a two-way conducting device which becomes conducting when a current of sufficient strength is applied to its gate electrode and which then remains conducting independently of the gate current from the moment when, and as long as, a current of a strength greater than a given threshold flows therethrough between its main electrodes.
Thus, in triac control circuits, possibly constructed in the form of integrated circuits, efforts are made to supply the gate electrode of a triac with a pulse of sufficient amplitude and duration to allow triggering thereof. Nevertheless, so as to avoid increasing the consumption of the control circuit, efforts are made to limit as much as possible said amplitude and said duration. Determination of the amplitude of the pulse to be applied is relatively simple and depends on the characteristics of the triac, more especially on the quadrants in which the control takes place, some quadrants being more sensitive than others. The choice of the duration of the control pulse is much more delicate for it depends on the characteristics of the load circuit in which the triac is inserted. In particular, if this load circuit has a relatively high self-induction, the time for establishing the current will be slow and it will be advisable to choose a pulse of considerable length, this length depending on the nature of the load circuit.
Thus, it is necessary, if it is desired to construct a triac control circuit able to operate with loads of different natures, to choose relatively high gate control pulse durations, the length of these pulses being superfluous in the case where the load has low or no self-induction. The result is an unnecessarily high energy consumption for the gate control circuit in this latter case.
Thus, an object of the present invention is to provide a process and a device for triggering a triac allowing low energy dissipation in the control circuit.
In the present invention, we will be more particularly interested in the conduction angle control for triggering a triac fed with a periodic current, for example at 50 Hz.
It is then a question of supplying a first pulse to the triac after a time depending on the chosen conduction angle, then in repeating successive pulses if the first pulse was not sufficient to produce triggering. A difficulty arises in that at the end of the conducting period, the triac shuts off, the result is a rise in voltage at the terminals of the triac and this voltage rise must not be interpreted as a triggering failure for this would lead to supplying one or more pulses at the beginning of a half-wave and so in effecting a "full angle" control.